Academics

Dr Stephen Clark

Career Development Fellow in Quantum Networks

Welcome

I currently hold a joint position as a Career Development Fellow and Tutor in Physics at Keble College and a Senior Research Fellow at the Department of Atomic and Laser Physics. For four years previously I was Research fellow at the Centre for Quantum Technologies in the National University of Singapore. My teaching duties at Keble mainly involve demystifying Quantum Mechanics to 1st and 2nd year undergraduates. For those students not deterred by this experience I also try to run a summer project each year to give them a flavour of research and to stretch their physical intuition.

Research Interests

My research focuses on what in physics-language are termed Strongly-Correlated systems. Broadly speaking such systems arise whenever the constituents which make them up possess significant interactions with each other. More often than not this results in the system exhibiting rich collective phenomena in which all of its parts behave in concert. Examples of system which fall into this category are far and wide. As I will describe below this is reflected in the diverse range of bodies which we may identify as the individual constituents, including particles in a lattice, vehicles on a road, data packets in a computer network, pathogens in a population and many more.

Within physics much attention is given to the case where the constituents are particles and evolve according to the rules of quantum mechanics. Collisions between these particles can cause these types of systems to exhibit many fascinating and counter-intuitive phenomena such as superconductivity and fractional excitations precisely due to their quantum nature. Such models are of great significance to major research areas like condensed matter and particle physics because they capture much of the underlying physics which is still not yet fully understood. Much of my research to date (see publication list below) has involved exploring quantum many-body physics of this kind.

In addition to these quantum systems my research now has an interdisciplinary component in which I investigate classical strongly correlated systems that describe highly relevant every-day effects. One example is a road traffic system. Here the constituents are vehicles and usually possess a “hard-core” interaction with each other. This interaction alone results in road systems exhibiting a wide range of phenomena such as the spontaneous formation of traffic jams, metastable flow, and even self-organized criticality. Anyone who regularly commutes into Oxford will readily recognise the former. Another example, currently topic due to Swine flue, is the spreading of a pathogen within a population. This can be modelled as a network where nodes represent people and connections represent personal contact. Interactions arise from the transition of an individual from healthy to infected via a connection to a diseased person. Naturally healthy people will try to alter their connections to avoid diseased individuals and this leads to phenomena such as "phase-separation" where health and diseased populations separate from one another, and "epidemic cycles" where the proportion of the population that is diseased flares up and dies down on a regular basis. The college environment with its annual freshers flu is perhaps a microcosm of these effects.

A crucial feature of all these systems, both quantum and classical, is that to compute their properties, even in this most idealized form, is a highly intractable problem. This is due to the exponential scaling, typical of many-body systems generally, in the number of parameters needed to describe the system as the number of constituents grows. The central goal of my research is overcome these difficulties by applying sophisticated algorithms, originally developed exclusively for quantum systems, which distill the core properties of the system into a tractable representation. Once accomplished high performance computing facilities here in Oxford will allow us to not only better understand fascinating many-body phenomena but also aid in the optimization of real-world strongly correlated systems through numerical experiments. While I can't promise this research will make traffic jams will be a thing of the past it might at least explain the variety of perplexing situations in which they seem occur.